Isozymes of hexokinase in Limulus polyphemus have been studied and compared with those of other invertebrates and vertebrates. Limulus has three distinct hexokinases, one similar to our recently described mannofructokinase of lobster, but totally lacking in the ability to react with glucose. In addition Limulus contains two sugar dehydrogenases, not found in lobster, capable of oxidizing various sugars without requiring prior phosphorylation by ATP. Isolation and characterization of these enzymes is being pursued. The metabolism of glucose in Limulus and its conversion to glycogen of muscle and hepatopancreas is being studied using specifically isotopically labeled glucose and glucose precursors.